The present invention relates to a method and a system for grouping relay stations in a wireless multi-hop relay communication system. More particularly, the present invention relates to a method for scheduling a wireless multi-hop relay communication system so as to improve the transmission efficiency and capacity of the wireless multi-hop communication system.
Next generation mobile communication systems may be envisioned to provide high-speed, high link quality, and high security transmissions, and may also be expected to support various communication services. An effective resource schedule/allocation method may have to be established to meet different quality of service (QoS) requirements from different users. Users located at cell boundary may have worse link quality due to the long transmission distance to a base station, and users in a cell with severe shadowing effect may also have worse link quality, thereby the foregoing users may not perform high-speed data transmissions. To resolve the foregoing problem, the deployment density of base stations may be increased to shorten the propagation distances between the base stations and the users so as to improve the link quality, or more base stations may be deployed at those areas with severe shadowing for improving the link quality of users in the areas. However, the cost of the base stations and the cost of the backhaul network connections may be substantially increased by the aforementioned method. On the other hand, the transmission power of the base station may be increased to improve the link quality and to reduce the cost of the base station. However, if the transmission power is increased, not only the transmission cost but also the interference level may be increased.
Multi-hop relay cell architecture may be a good solution when considering all factors such as QoS, deployment cost, transmission power, and coverage area of the cell. Relay stations may be deployed within a cell to relay information from a base station to mobile stations with worse link quality, and vise versa. It has been shown that using relay stations may improve cell coverage, user throughput and system capacity.
Relay stations may be deployed at areas with severe shadowing or near the cell boundary, the users who may not be directly served by base station may be served by the relay stations, therefore the effective coverage area of the base station may be extended.
A single link with worse quality may be divided into a plurality of links with better quality so that each of the links may provide higher transmission rate. However, since the same data may be duplicated and relayed over the air multiple times for multi-hop transmissions, it may consume the radio resources.
Moreover, since there may be a base station and several relay stations in a cell, to improve the spectrum efficiency, multiple serving stations may be active simultaneously if the potential interference is tolerant.
To obtain benefits for multi-hop relay communication systems, an efficient scheduling mechanism may require arranging the transmissions of base stations and relay stations.
To improve the performance of a wireless communication system, a method of relay stations deployment in a Manhattan-like environment was provided in the Wireless World Initiative New Radio (WINNER) program. The Manhattan-like environment is a grid environment wherein the width of blocks is about 200 meters (m) and the width of streets is about 30 m.
FIG. 2 is a diagram illustrating a first layout of a base station 205 and a plurality of relay stations 201 to 204 of a single cell in a Manhattan-like environment in a conventional communication system. Referring to FIG. 2, the base station 205 and the relay stations 201 to 204 are disposed in the single cell, and the base station 205 and the relay stations 201 to 204 may all communicate with users through omni-directional antennas. However, since the relay stations 201 to 204 may be disposed outside a coverage area 206 of the base station 205, each of the relay station 201 to 204 may require an additional directional antenna pointing at the base station 205 for communicating with the base station 205, and thus may increase the hardware cost of the relay stations.
FIG. 3 is a diagram illustrating a transmission scheduling for a frame structure applicable to the first layout shown in FIG. 2 within a single cell in the Manhattan-like environment. Referring to FIG. 3, a frame S301 may be divided into two sub-frames S302 and S303. The first sub-frame S302 may further be divided into 5 time slots S304 to S308, wherein a base station 305 may serve four relay stations 301 to 304 during the first four time slots S304 to S307, respectively, and the base station 305 may serve users within an area 306 which may be directly connected to the base station during the fifth time slot S308. The second sub frame S303 may be divided into two time slots S309 and S310, and with the characteristics of spatial separation of the environment, the relay stations 301 and 302 may serve users within areas 307 and 308 connected thereto during the same time slot S309, and the relay stations 303 and 304 may serve users within areas 309 and 310 connected thereto during another time slot S310.
FIG. 4 is a diagram illustrating a layout of base stations 405, 415 and relay stations 401 to 404, 411 to 414 in a multi-cell structure in the Manhattan-like environment illustrated in FIG. 2. Referring to FIG. 4, a coverage area 406 of a single cell A and a coverage area 416 of a single cell B are arranged in a staggered way. Moreover, the base stations 405 and 415 in FIG. 4 respectively represent the positions of the base stations in the single cell A and the single cell B, the relay stations 401 to 404 belong to the single cell A, and the relay stations 411 to 414 belong to the single cell B.
FIG. 5 is a diagram illustrating a transmission scheduling for a frame structure applicable to the layout shown in FIG. 4 within the multi-cell structure in the Manhattan-like environment. Referring to FIG. 5, an arrangement of transmission frames between adjacent cells may be used to permute the operation orders of sub-frames S502 and S503 in a frame S501 so that interference between cells may be prevented. The main purpose of the relay stations may be to extend the coverage area of the base station. However, the link quality of users at the boundary of the service range of the base station may not be improved. Moreover, all of the base stations may be idle for some time durations in the frame structure. Since base stations may be the only serving stations connected to the backhaul networks and carrying the effective data, the transmission efficiency of the base station in this design may not be desirable.
FIG. 6 is a diagram illustrating a second layout of a base station 605 and four relay stations 601 to 604 with omni-directional antennas in a Manhattan-like environment. Referring to FIG. 6, the base station 605 and the relay stations 601 to 604 may all communicate with users by using omni-directional antennas. Since the relay stations 601 to 604 are disposed within a coverage area 606 of the base station 605, no additional directional antenna may be required by each of the relay station 601 to 604 for communicating with the base station 605. With the design, the link quality of users in the cell boundary may be improved.
FIG. 7 is a diagram illustrating a transmission scheduling for a frame structure applicable to the second layout shown in FIG. 6 with all serving stations equipped with omni-directional antennas in the Manhattan-like environment. Referring to FIG. 7, the base station 705 may serve four relay stations 701 to 704 sequentially during first four time slots S701 to S704, and at the same time, the base station 705 may serve users directly connected to the base station 705. The relay stations 701 and 703 may serve users connected thereto during the time slot S705. After that, the relay stations 702 and 704 may serve users during the next time slot S706. The main purpose of such a layout may be to improve the link quality of users at cell boundary. However, a complete transmission within a single cell may require at least 6 phases to be completed. When considering the multi-cell structure, because of the use of omni-directional antennas, the reuse factor of at least 2 may be required to avoid the severe inter-cell interference, and thus decreases the overall system capacity.
Regardless of the first layout or the second layout that all serving stations are equipped with omni-directional antennas, all the base stations and the relay stations may be idle for some time in the frame structure. Accordingly, the transmission efficiency thereof may not be desirable. It may therefore be desirable to have a scheduling method for a wireless multi-hop relay communication system for improving the transmission efficiency and capacity of the system.